How long does the tin foil need to be around the crust when baking?
It helps if you think about the reason the foil is used in the first place:
If a pie filling needs a certain time to set or bake, the crust might brown too much or even burn. The foil partially shields the crust from the oven heat.
How pronounced the “crust getting too dark before filling is done effect” really is depends on the recipe (combination of type of filling and type of crust) and the oven temperature. The characteristics of your oven are also to be considered. So giving you a formula is not effective and may even be plain wrong.
What you should do is:
- If the recipe gives a recommendation, stick to that, adjust as needed if the crust browns too quickly or not enough.
- Otherwise, observe the pie and if you notice that the crust seems “almost right”, but the filling needs obviously longer than until the crust is done, add the foil shield.
I encourage you to start baking without the foil and add it according to visual clues. If you have enough experience with a specific recipe and know quite precisely how long to bake with or without foil, you can also shield first and remove the foil later, if you really want to.
On average, you will want to remove the tin foil 20 minutes before it's done. That has been the rule I have followed.